The activity of the perioral muscles during breastfeeding in infants was investigated using EMGs. Fifty-six infants aged from 1 to 5 months were classified into five groups according to month of age in the cross-sectional study. Follow-up was carried out on 18 infants whose mean age was 2.5 months at the initial examination, and 4.8 months at the second. During suckling, EMGs were recorded unilaterally from the temporalis (TM), the masseter (MM), the orbicularis oris (OM), and the suprahyoid (SM) muscle groups. The activity of the SM increased significantly with age, while there was no appreciable increase in the activity of the TM, MM, and OM in either the cross-sectional study or the follow-up. However, total muscle activity was shown to increase significantly in both parts of the study. These findings suggest that the active tongue- and jaw-lowering movement may play a primary role in increasing sucking strength during the suckle-feeding period in infants.